The downforce generated by a vehicle can have a substantial effect on the cornering and braking capability of the vehicle. In practical situations, it is often the case that an increase in downforce (by, for example, adding spoilers to the vehicle) is accompanied by an increase in drag, meaning that a vehicle set-up to generate high levels of downforce will be compromised by a reduction in its maximum straight line speed.
FIG. 1 is a schematic diagram of the airflow over a typical vehicle. Taken in the frame of reference of the vehicle travelling in the negative x-direction, as the air flows over the vehicle it will stagnate at its front end 101 creating a region of relatively high pressure. The fast flowing air over the bonnet will create a region of relatively low pressure 102. The sharp change in curvature of the body of the vehicle towards its rear end 103 will cause the flow to separate from the body in this region, creating a region of low pressure 104. Various aerodynamic devices have been developed to increase the pressure in the low pressure regions and to decrease the pressure in the regions underneath the vehicle in order to increase the downforce generated by the vehicle and improve cornering performance.
A common device used to increase the pressure of the air in the low pressure region is the rear wing. A typical rear wing is shown in FIG. 2. The profile of a typical rear wing consists of a relatively flat upper portion 201 and a relatively curved lower portion 202. As air flows over the wing the curved lower portion deflects the air upwards, hence the wing is exerting a resultant force F1 on the air in the positive vertical direction. In accordance with Newton's third law, the air will exert an equal and opposite force on the wing. This equal and opposite force F2 is the downforce generated by the rear wing.
A splitter is an aerodynamic device that operates to decrease the pressure of the air under the body of the vehicle. A vehicle fitted with a typical splitter is shown in FIG. 3. As air travels towards the front of the vehicle a portion of the airflow will be directed over the top surface of the splitter 301 and a portion of the air will be diverted under the bottom surface of the splitter 302. The airflow over the top of the splitter will stagnate, causing a region of high pressure, whilst the portion of the airflow that is channelled between the splitter surface 302 and the ground will be accelerated under the body of the vehicle. For a particular streamline, increasing the flow speed of the air will cause its pressure to drop in accordance with Bernoulli's principle, and so channelling high speed air under the surface of the vehicle helps to lower the pressure in this region and hence increases the downforce generated by the vehicle. This effect is increased as the distance between the lower surface 302 of the splitter and the ground is decreased.
Louvered wheel arches can also be used to increase the downforce generated by a vehicle and are typically implemented on race cars. An example of a louvered wheel arch is shown in FIG. 4. When a vehicle is travelling at speed the pressure of the air inside the wheel arches 401 is generally higher than the pressure of the air flowing over the surface of the vehicle body 402. Louvered wheel arches 403 alleviate this result by providing a path for the high pressure air inside the wheel arches to flow into the lower pressure region over the surface of the body. Louvered wheel arches are also useful for aiding the cooling of the brake system.
It is often desirable for a vehicle to increase the downforce it generates due to the beneficial effects on its cornering ability. However, increasing the downforce can lead to an increase in drag which compromises the straight line speed of the vehicle and increases its fuel consumption. Furthermore at very high speeds the downforce generated may overload the vehicle and result in significant changes in aerodynamic stability under different dynamic conditions, making the vehicle difficult to control.
There is thus a need for an improved means of controlling the downforce generated by a vehicle.